A single-mode optical fiber transports light at small angles relative to the optical axis of the fiber. Unfortunately, light sources such as edge-emitting semiconductor lasers that provide optical signals for transmission on optical fibers often have large-angle light patterns that are difficult to efficiently couple into a single-mode optical fiber. Prior solutions for coupling the light from edge-emitting semiconductor lasers into optical fibers have included direct “butt-coupling” and use of a lens to image the laser light onto the input face of the fiber.
Butt-coupling involves placing the input face of an optical fiber at or near the laser output aperture without additional intervening optics. This approach often captures only a small fraction (typically less than 10%) of the laser power. Butt-coupling is also intolerant of small relative movements or misalignments between the laser and the fiber. For a single-mode fiber having a core that is about 9 μm in diameter, a relative movement on the order of 1 μm can cause large changes in the amount of coupled power.
A lens between a laser and an optical fiber can image the laser light onto the fiber core to capture a larger fraction of the laser power. With a high magnification, the lens can convert the large-angle light pattern from the laser into a small-angle pattern that has a near 100% coupling into the optical fiber. However, a high magnification imaging system is very sensitive to relative misalignments of the laser, lens, and optical fiber. For example, for a 5× magnification, a lateral laser-to-lens shift or misalignment by a distance Y shifts the imaged light by 5Y, and an axial laser-to-lens shift or misalignment by a distance Z shifts the imaged light off the fiber by a distance 25Z.
The strong misalignment sensitivities of optical fiber couplers for single mode optical fibers have required aligning the laser, the lens, and the fiber to accuracies less than 1 μm along all three axes. Further, to maintain a consistent and adequate power coupling, the precise alignment in an optical fiber coupler must be maintained despite jarring that may occur during assembly, integration, or use of the optical fiber coupler and despite thermal expansion of the elements and packaging of the optical fiber coupler. Achieving and maintaining the necessary alignment accuracies can be time consuming and expensive tasks. For high-volume, low cost manufacturing, coupling methods and structures are needed that provide a high and consistent power coupling efficiency while tolerating larger alignment errors.